Elastic modulus of calcified cartilage is an order of magnitude less than that of subchondral bone

The elastic moduli of calcified cartilage and subchondral bone tissues were measured experimentally with use of a three-point bending test. Specimens were obtained from a bovine patella and the distal end of a bovine femur, from two different animals. Fifteen specimens were tested as "pure" subchondral bone beams, and 15 were tested as composite calcified cartilage/subchondral bone beams. A least-squares optimization scheme was used to obtain modulus values from the composite beams. The elastic modulus for subchondral bone calculated from the "pure" subchondral bone beams was 2.3 +/- 1.5 GPa (3.9 +/- 1.5 GPa for specimens from the femur and 1.6 +/- 0.7 GPa for specimens from the patella). The composite beam optimization resulted in a modulus for subchondral bone of 5.7 +/- 1.9 GPa and a modulus for calcified cartilage of 0.32 +/- 0.25 GPa. The modulus for the calcified cartilage was more than an order of magnitude lower than the modulus of the underlying subchondral bone. This supports the idea that the zone of calcified cartilage forms a transitional zone of intermediate stiffness between the articular cartilage and the subchondral bone.